A workpiece chuck can be used to hold workpieces such as semiconductor wafers during processing and testing. Because integrated circuits formed in a wafer under test are commonly tested over temperature, the workpiece chuck can include a temperature control system for controlling the temperature of the wafer during testing. As integrated circuits become smaller and more densely integrated, positioning tolerances for testing systems such as wafer probers decrease. With very small positioning tolerances, it is very important that the chuck supporting the wafer during testing be mechanically stable. This requirement is especially challenging in a temperature-control chuck where the chuck must maintain mechanical and electrical stability over a wide range of temperatures. At temperature extremes, particularly at high temperatures, conventional wafer chucks tend to distort due to thermal expansion and contraction and the integrity of the materials of which the chucks are constructed. For example, chucks made of softer materials will tend to distort at high temperature. This problem is exacerbated by the increasing mechanical loads on the chuck introduced by the test system, i.e., the wafer prober.
The present invention is directed to various aspects of a workpiece chuck which overcome drawbacks of conventional chucks to provide a chuck with improved mechanical and electrical stability. According to a first aspect of the invention, there is provided a thermal control apparatus, or thermal plate assembly, and method, which can be used to control temperature in a workpiece chuck. The thermal control apparatus includes a heating element and a cooling element. The heating element is disposed in a heating plane, and the cooling element is disposed in a cooling plane. The heating plane and the cooling plane can be coplanar, i.e., they are the same plane.
The thermal plate assembly of the invention can be a layer in the workpiece chuck. Because the heating and cooling elements occupy the same horizontal plane of the chuck, the heating and cooling are uniform across the top surface of the chuck where the workpiece, i.e., wafer, is supported. Also, because the heating and cooling elements are coplanar, substantial distortion and warping of the chuck and workpiece over temperature are eliminated.
The heating element can include an electrical resistive heating coil element. The heating coil element can be disposed in the heating plane in a coiled configuration.
The cooling element can include one or more hollow tubes for circulating a temperature-controlled fluid through the thermal plate assembly. The circulating tubes can be disposed in the cooling plane in a coiled configuration.
To facilitate locating both the heating element and the cooling element in the same plane in coiled configurations, the heating and cooling elements are spatially interleaved with each other. The interleaved nature of the heating and cooling elements also provides more uniform heating and cooling of the chuck and, therefore, more uniform temperature across the surface of the wafer. Also, warping and other distortion of the chuck over temperature are substantially eliminated, such that the chuck can hold the wafer extremely flat over temperature.
In one embodiment, warping and other distortions over temperature are further reduced by the selection of the location of the heating and cooling plane within the thermal plate assembly. In this embodiment, the heating and cooling planes are located in a center plane of the thermal plate assembly, i.e., the plane that is equidistant from the top and bottom surfaces of the thermal plate assembly. With the heating and cooling planes located at the vertical center of the thermal plate assembly, distortions caused by doming and/or dishing of the thermal plate assembly are substantially eliminated. Again, with the reduction in chuck distortion over temperature, the wafer can be held flat over temperature.
In one embodiment, the thermal plate assembly of the invention is made from a cast material, which, in one particular embodiment, is aluminum. The casting of the housing provides the thermal plate assembly with improved mechanical rigidity and stability over temperature. The housing casting can be stress relieved such as by heat treating at predetermined manufacturing steps. For example, stress relieving can be performed both before and after finish machining of the housing. The stress relief provides the housing with more mechanical stability over temperature. Also, the housing casting can be formed with the tubes for circulating the cooling fluid. With the stress relief procedure, even more mechanical stability is provided.
In another aspect, the invention is directed to a workpiece chuck capable of implementing interchangeable top surface assemblies. In accordance with this aspect, the workpiece chuck of the invention includes a lower support and the thermal plate assembly on the lower support. The top of the thermal plate assembly includes a mount apparatus capable of holding multiple types of top surface assemblies, which are used to hold the workpiece/wafer to the chuck.
This configuration provides the chuck of the invention with flexibility according to the setting in which the chuck is being used. For example, one type of top surface assembly may be required where the test being performed requires the chuck to be able to absorb a large amount of power. In another test, the top surface assembly may be required to provide low electrical capacitance, high voltage or high electrical isolation performance. In still another test, the top surface assembly may be required to provide for very low signal leakage. In each of these tests, the top surface assembly may be fabricated differently to provide optimal performance under the specific testing parameters. In a conventional chuck system, this would require the user to obtain several different chucks, one for each test type. However, in accordance with this aspect of the invention, the thermal plate assembly of the invention provides a universal type mount which can accommodate all of the various top surface assemblies. As a result, the user need only have a single thermal plate assembly. The user can then purchase only the top surface assemblies required for the tests to be performed. This results in considerable cost savings to the user.
In another aspect, the invention is directed to an approach to eliminating the negative effects resulting from the relative movement of layers of the workpiece chuck over temperature. When two adjacent chuck layers have different thermal expansion coefficients, they tend to rub each other over temperature. This can cause abrasion of the surfaces which can degrade chuck performance. This is especially true in one particular example where a top surface assembly made of a hard abrasive ceramic material is located adjacent to the cast aluminum housing of the thermal plate assembly. To reduce these effects, one or more adjacent surfaces can be coated with a hard coating, such as hard anodize.
In accordance with this aspect, the invention includes a lower support and the thermal plate assembly mounted on the lower support. An upper support, e.g., top surface assembly, by which the workpiece can be mounted to the chuck, is mounted over the thermal plate assembly. A hard coating layer, a hard anodize layer for example, is adhered to a surface and interposed between the thermal plate assembly and the upper support assembly.